1. Field of the Invention
The present invention relates to a semiconductor device constituted by thin film transistors (TFT) formed on a substrate having an insulating surface and a method of manufacturing the same. Such a semiconductor device includes an IC, an LSI, an electrooptical device and the like, and especially it is effective to apply the present invention to the electrooptical device.
2. Description of the Related Art
In recent years, research in a semiconductor device constituted by TFTs using polycrystalline silicon thin films (polysilicon film: p-Si film) instead of TFTs using noncrystalline silicon thin films (amorphous silicon film: a-Si film), is developed. Especially, development and research in an active matrix type display device in which semiconductor devices are integrated on a substrate, is remarkable, since the demand as a display device of a PC monitor, a video camera, a projection or the like has been increased.
As such an active matrix type display device, there is known one disclosed in U.S. Pat. No. 5,250,931 (Misawa et al.). However, additional values are recently required, and there has been proposed an SOP (System On Panel) plan in which a logic circuit (signal processing circuit such as a display control circuit and an arithmetic circuit), which are conventionally realized by an externally equipped IC, is mounted on the same substrate by TFTs.
Also, research for manufacturing a conventional IC or VLSI itself by TFTs, is made to proceed. It is because a TFT is formed on an insulating substrate so that it has a feature that parasitic capacitance is extremely small, and it can be said that the TFT is more effective in high speed operation than an FET formed on single crystal silicon.
However, a technique for forming a TFT which is capable of realizing the plan has not been established. The reason is that when a presently used polysilicon film (including a so-called high temperature polysilicon and low temperature polysilicon film) is employed, it is difficult to obtain a TFT having enough performance to constitute a circuit requiring high frequency driving.
Although the improvement of an operation speed of a TFT can be made for the time being by decreasing the size of the TFT, the reduction of the channel length (or gate length) results in the short channel effect, so that disadvantages such as lowering of a drain withstand voltage is caused. Thus, in the TFT using a conventional silicon thin film, the improvement of an operation speed by the scaling law reaches the limits, and it is difficult to increase the operation speed any more in view of reliability. Further, the silicon thin film has a problem that crystal grains and crystal grain boundaries exist irregularly, and the crystal grain boundaries greatly influence the TFT characteristics to cause irregularity.